unfortunately, that's just how it works now. you used to have the option with australia post's label system to print labels and then pay at the post office when you lodge it but this no longer is the case.
everyone else is in the same boat unless they have their own arrangements in place.
if you need to, come up with a flat rate postage amount for the item by weighing it when you list it and allowing for the size and additional weight of packaging.
otherwise, the convenience factor aside, you can buy your own packaging materials from wherever it may be and then use a normal address label and take it to a post office to lodge it.
the convenience isn't conditional - it's a matter of "fit in the box" or don't use it (no pun intended).
